1. Field of the Invention
The present invention relates to a pressure sensor, and more specifically, to a pressure sensor which has a metal diaphragm and transmits pressure through fluid and is used to sense pressure, for example, when combustion pressure in an engine is measured.
2. Description of the Related Art
FIG. 3 is a cross sectional view showing the structure of a conventional pressure sensor used to measure combustion pressure in an engine, and the like, wherein a generally cylindrical hollow case 1 made of stainless steel such as SUS 304 or the like has a threaded portion 2 formed to the periphery of the case 1 at the lower portion thereof to mount the case 1 on an engine or the like (not shown). The case 1 has a substantially columnar central bore 3 extending therethrough which has a shoulder portion 3a at the middle portion thereof. A stem 7 made of mild steel is disposed on the shoulder portion 3a and a pressure sensing element 5 composed of a semiconductor strain gauge is attached to a glass base 6 adhered to the lower surface of the stem 7. Further, output lines 8 extend through the stem 7 and are electrically connected to the pressure sensing element 5 through bonding wires 9. A waveform or corrugated diaphragm 4, which is made of stainless steel SUS 304 or the like and has a thickness of about 40 .mu.m and a diameter of about 8 mm, is attached to the lower end of the case 1 by, for example, welding, and a pressure transmitting medium 10 composed of liquid such as silicone oil or the like is filled within the central bore 3 between the stem 7 and the diaphragm 4 for transmitting a pressure to be detected.
Next, operation will be described. The diaphragm 4 is disposed in the combustion chamber of an engine or in a pressure transmitting path connected to the combustion chamber. The diaphragm 4 is deformed in accordance with a pressure variation in the combustion chamber, and transmits a pressure to the pressure sensing element 5 through the pressure transmitting medium 10. The transmitted pressure variation in the combustion chamber is converted into an electric signal by the pressure sensing element 5 and the electric signal is output through the lines 8 electrically connected by the bonding wires 9.
The conventional pressure sensor has a problem that although there is a difference of frequency between an output resulting from a pressure variation in a normal operation and an output produced when an abnormal pressure vibration such as knocking and the like is caused, there is no difference between the magnitudes or the of the outputs. Therefore, since it is difficult to discriminate between normal and abnormal, outputs the occurrence of the abnormal pressure vibration such as knocking and the like is difficult to be sensed.
It should be noted that since the conventional pressure sensor uses a diaphragm having a diameter of about 8 mm and a thickness of about 40 .mu.m and has a resonance frequency of 10000 Hz.about.10300 Hz and further a pressure vibration frequency in the combustion chamber is 5000 Hz.about.9000 Hz when the abnormal pressure vibration such as knocking and the like occurs, even if the abnormal pressure vibration such as knocking and the like occurs, the diaphragm does not resonate similarly to the case of normal operation and an output having a large gain cannot be obtained. Therefore, the occurrence of abnormal pressure vibration which is different from that occurring in normal operation is difficult to discriminate.
An object of the present invention made to solve the above problem is to provide a pressure sensor having a diaphragm which is resonated in response to abnormal pressure vibration such as knocking and the like in order that the occurrence of abnormal pressure vibration such as knocking and the like can be easily sensed by the resonance of the diaphragm.